Articulations have been known for a long time which are used either to adjust inclination of the backing portion of a vehicle seat, or to adjust height of the sitting portion of this seat.
These articulations are of two kinds, mainly either articulations with two elongated flanges for enabling fixation of the articulation i) on the fixed portion of the seat in consideration and ii) on the mobile portion or, on the contrary, articulations with circular flanges having an outer surface with protrusions enabling fixation of these circular flanges alternately on the fixed portion or on the mobile portion of the seat. The mechanisms used in these articulations are all known and belong either to the "continuous" type articulations with or without a satellite, or to the "discontinuous" type articulations, i.e. with at least one sliding block, and generally three sliding blocks, placed at 120.degree. with respect to one another and cooperating with an inner toothing forming a mobile ring.
The articulations with elongated flanges have a great disadvantage insofar as in certain extreme angular positions of one flange with respect to the other flange, the stability of the two flanges is not ensured.
The articulations with round flanges comprise a closing ring providing for a good stability of the flanges whatever is their angular position. But unfortunately such flanges are difficult to make interdependent in particular with a structure formed of a tube.